In the fabrication of microelectronic devices such as integrated circuits, an electronic substrate such as a wafer must be processed in numerous processing steps, which in some cases may include as many of several hundred processing steps. During each of the processing step, the silicon wafer must be transported in and out of specific process machines such as an etcher, a physical vapor deposition chamber, a chemical vapor deposition chamber, etc. Between the processing steps, a preprocessed wafer is stored in a storage container referred to as a wafer cassette. The wafer cassette is then stored in a container known as a pod to prevent contamination.
The wafer cassette is a device that is normally molded of a plastic material which can be used to store a large number of wafers in a horizontal position. In order to maximize the number of wafers that can be stored in a cassette, the wafers are positioned relatively close to each other. For instance, the pitch distance between the wafers is approximately 2 mm in a normal cassette. The wafers, when stored in the cassette are supported along the wafer edges by molded-in supports on the inner walls of the cassette.
In order to load a wafer into or out of a process machine or wafer cassette, a device known as a wafer transport blade, wand or end effector is typically used. An end effector is a thin piece of material that may be formed in any of a variety of shapes but preferably includes a base portion with extensions commonly known as fingers. An end effector is an attachment to a robotic arm that is used to transport silicon wafers, hard drive disk, or flat-panel substrates from one location to another. The end effector can be supplied for vacuum or non-vacuum applications, and may also include wafer edge gripping clamps. An end effector may be made from ceramic materials such as alumina or silicon carbide, or a metal such as aluminum.
As indicated earlier, the processing of the silicon wafer involves moving the wafer from a cassette to various processing locations by a robotic handling system. The robotic handling system includes a mechanism with degrees of freedom in at least radial, angular and vertical directions with the end effector attached to the end of a robotic arm. The robotic arm must be able to pick up the wafers from the cassette and then transfer them to the designated stations where the wafer undergoes a variety of process steps. The robotic mechanism and its associated controller must be programed with the precise location in terms of radial, angular and vertical positions of the wafer in all cassette locations and all processing station locations. A robotic mechanism controller, such a central processing unit, includes programmed data to locate and retrieve the wafer precisely from the cassette or processing station.
In a typical wafer processing layout, the locations of various process stations and the cassette stand are known, and the dimensional relationships between the wafer positions in the cassette, each process station location and the robotic arm are known within macro-tolerances, for example within 0.05 inches. However, the robotic arm must be controlled to move the wafers within extremely close tolerances, that is within micro-tolerances, in order to prevent damage to the robotic system including the end effector, wafers, the wafer cassette or other semiconductor processing equipment.
Typically, the robotic system is set up and pre-programmed with the location of the wafer positions within the cassette stand, the location of the cassette stand, and other process stations using macro-tolerances. Thereafter, the robot must be programmed or taught so that the robot arm and end effector are precisely positioned during each of the operation steps within micro-tolerances. A typical way of accomplishing this is to manually move the robotic arm and end effector to each location within the wafer handling process and make adjustments to the robotic mechanism and control system. Once the end effector is properly located for each process handling step the precise positions are then stored in the memory of the wafer handling mechanisms controller.
However, occasionally a piece of the processing equipment such as a wafer cassette may not be precisely positioned within specifications, or the machine components wear, settle, malfunction, or components are replaced resulting in the robotic control arm not being able to move to precisely the correct position for handling the wafer. Often, such situations require the robotic mechanism to be reprogrammed to accommodate the new changes and new locations. However, in the case of component wear, equipment settling or malfunction, the robotic mechanism operator is not aware of the situation until one of the process equipment components or the end effector is damaged. The end effector is often damaged by bumping into the wafer cassette or the wafer itself resulting in the end effector finger extensions being broken off.
Thus it would be desirable to provide a wafer handling system that would be able to accommodate component locations that are mis-programmed into the wafer handling controller, changes in the location of the robotic arm, end effector or other processing components due to wear, settling, or malfunction in the same without resulting in damage to the wafer processing components including the end effector. The present invention provides alternatives to and advantages over the prior art.